1. Field of the Invention
The present invention relates to a roof assembly for a building structure, and more particularly, but not by way of limitation, to roof assembly improvements providing greater load bearing capabilities.
2. Discussion
An exposed metal roof has become the preferred roof for most commercial and industrial building systems in the USA. An exposed metal roof typically comprises steel metal panels anchored to structural support members, the metal panels having a selected thickness. The most common thicknesses are 22, 24 and 26 gauges. The metal panels are formed by various means to provide structural strength sufficient to resist roof traffic, live load (snow and pounding water) and wind loading, which results in uplift.
Steel panels provide a widely accepted weather membrane. If coated with a whether resistance coating the panels can last 30 to 40 years with little maintenance. Steel panels in sheet form are not practical for roofing without some forming to provide resistance deformation by live loading, including wind loads. This typically consists of forming corrugations in the steel sheets to form roof panels. Typical panels are 12 inches to 36 inches wide, depending on the type of corrugation and type of roof.
Metal roofs can be classified into two broad types, corrugated screw down roofs and standing seam roofs. Screws down roofs are roofs which are secured to supporting members by fasteners penetrating the panels. Screws down roofs are typically composed of panels with corrugations at 3 to 6″ intervals and are attached to each other; fasteners penetrate the steel panels in the flat between corrugations and along the longitudinal sides of the panels to each other.
Standing seam roofs are attached to underlying supporting structurals by means of clips encased into the seam and attached to the building structurals, the seams being mechanically seamed with the edge seams so that the steel panels are not punctured with fasteners. The present invention is applicable to such standing seam roof systems.
Standing seam roof panels are formed into various shapes but fall into two broad types, pan panels and trapezoidal panels. Pan panels have a vertical leg corrugation at each longitudinal edge of the panel; trapezoidal panels have a trapezoidal corrugation at each longitudinal edge of the panel. Both systems have a seam formed into the top of the corrugations configured to join overlapping panel sides together, and after joined, the seam and corrugation will transfer loads from the panels to the structure.
The panels are attached to the building by means of clips placed on the seams and attached to underlying support structurals. After the seams of two or more panels are joined and clipped to the support structurals, with the clips attaching between the seam elements of adjacent panels, the panel seams are field formed into what is commonly referred to as a lock seam by an electrically powered seamer. The seamer folds the panel edges together, along with the clips, to form a lock seam of various configurations that resists separation of the panels.
This method of attaching panels to each other and to the structural system of the building eliminates the necessity of puncturing the weather resistant steel panels to attach the panels to each other and to the structural system, thus providing a water tight roof with no panel penetrations, avoiding water entry while still providing secure attachment of the roof panels to the support structures.
Because the panels are only attached at their sides only the forces of uplift load and live load are concentrated on the panel corrugations seams. This causes the panel to deflect in the center between longitude seams, and as the panel center is deflected the side corrugations are pulled apart and the seam deflects between clip attachments points, which can result in a failure of the roof panel by pulling the seam apart and or by excessive deflection between building structural attachment points.
Several industry tests determine the strength of a standing seam roof. One such test is the ASTM E 1592 test for uplift; another test is the Factory Mutual FM 4471 test for negative load (wind uplift); and other tests are used for the determination of deflection under live load. In this country, the most severe loading is that of a negative load. During tests for negative loading the panel flat deflects upward as much as 6 inches, placing severe stress on the panel seam. Live loads also deform the panel and stress the seam in a similar but less severe deflection. The result is that the seam unfurls and deflects between spans and failure occurs by buckling of the panel corrugation and seam and separation of the seam. To minimize this several methods have been used and are common in the industry.
One method to improve load capacity of roof panels is to reduce the span of the panels, that is, reducing the distance between underlying support structurals. This is effective to a point, but does not strengthen the panel seam from separation due to unfurling, and is costly because more secondary structurals are required over the entire roof.
Another method common in the industry is to increase the thickness of the panel material from 24 gauge to 22 gauge. This strengthens the seam and does reduce the seam unfurling and some deflection between spans. This method is not cost effective as the material for the entire panel is increased only where the seam and clip attachment points need to be reinforced, and the improvement in the seam strength is minimal.
Another method is to reduce the width of the panel, for example, from 24 inches to 12 inches. This is not cost effective because this requires additional clips, and since each panel has vertical or trapezoidal corrugations at the sides of the panels, additional panel material is required per square foot of roof.
Yet another method that is frequently used to strengthen panels is to attach a clamp over the outside of the corrugation at clip locations. This method does a good job of minimizing the unfurling of the panel seam at clip locations but does not minimize deflection of the panel seam at mid points or deflection either side of the clamp. The clamp will only keep the panel seam from unfurling at the clip location and will not improve defection between clips. The clamps, installed after seaming, are expensive and generally considered unsightly for many applications.
In addition many deficiencies, there is a need for improved means of strengthening standing seam roof assemblies to increase load bearing capabilities.